When performing accelerated lifetime testing of photovoltaic (PV) modules, one important degradation mode to consider is System Voltage Induced Degradation (SVID), also known in the art as Potential Induced Degradation (PID). SVID is a phenomenon that occurs when the cell circuits of the PV modules, such as the PV modules located towards the end of a string, may be at a relatively high potential difference relative to the frame of the module. This can lead to formation of a strong potential field within the module and concomitant generation of leakage currents through the module packaging materials. The extent of this leakage current is an important parameter to monitor as it is often an indicator of the extent of SVID.
SVID affects PV modules to varying degrees depending on the type and design of solar cells and the PV module packaging materials, which include the front cover, encapsulant and back cover of the PV module. The kinetics of degradation is generally driven by environmental conditions, such as temperature, relative humidity, moisture content. SVID is a long-term degradation mode, and presently there is no International Electrotechnical Commission (IEC) test that can evaluate the long-term performance of PV modules as it relates to SVID.
Certain known techniques have attempted to study SVID in PV modules, either in a controlled environment inside a chamber; or in the field by applying a voltage between the cell circuit and the frame of the module. The amount of leakage current flowing through a given PV module would be in direct correlation with the extent of SVID observed in the given PV module.
The present inventors have recognized certain limitations in connection with known techniques, which attempt to characterize SVID in PV modules by measuring the total leakage current flowing from the cell circuit to the frame of the module. Accordingly, such techniques can only be used to measure the lumped insulation resistance of a PV module as a whole, as a function of time. Therefore, one basic limitation in connection with such known techniques is that it is difficult, if not altogether unfeasible, to determine the individual contributions of the leakage currents throughout different leakage paths formed in the various materials, surfaces and/or interfaces in a PV module.